1. Field of the Invention
The present invention relates to railway switch mechanisms, particularly to railway switch mechanisms which operate to move mechanical rail portions, thereby transferring train traffic between alternate tracks, through motive force from electric motors, and more particularly to railway switch mechanisms which receive motive force from linear induction motors.
2. Description of the Prior Art
Current rail switch mechanisms use the established principles of mechanical advantage through devices such as gears, cranks, and lever arms to direct the path of the wheels of a train from one set of tracks to an alternate track. In addition, such mechanical devices are generally motivated by electrical and/or pneumatic linear/rotary actuators. Some such switch mechanisms uses a DC motor and a high-torque gear box (sideways worm-gear, screw-jack or spur-gear arrangement), and lubricated rail-support pads, which can require maintenance. In such switching mechanisms, a substantial portion of the actuator effort is directed to overcoming the effects of static friction and resistance, including coulomb and viscous friction forces. In addition, the actuator must be powerful enough to crush any ballast, snow or foreign matter that may have become lodged between the switch points.
The railroad industry has promulgated recommended operating guidelines for power-operated switch mechanisms which are to be met by existing and new switch mechanisms. Once such guideline is illustrated in FIG. 1. Typically, switch points move approximately six inches from one track to the other track. FIG. 1 indicates that an initial breakaway force of 400 pounds is required to overcome forces such as static friction in the system. During the next four inches of switch travel, torque is increased approximately linearly to about 900 pounds to overcome other oppositional forces such as friction and other viscous forces. To ensure switch point closure, the actuator is required to increase force on the switch to approximately 2500 pounds over the last one inch of travel to crush any matter such as ballast which may be entrapped between the main track and the switch.
Another switch design criteria requires that low voltage mechanisms with 20 volts at the motor terminals and high voltage mechanisms with 110 volts at the motor terminals must be capable of pulling 3800 pounds at end of stroke without damage. The switch mechanism must be designed so that it can be stopped, reversed, or obstructed at any point of its movement without damage. In addition, the switch mechanism must prevent movement due to vibration or external forces applied to the connections. Further, the switch mechanism in the locked position must be capable of withstanding stress equivalent to a thrust of 20,000 pounds either on the switch operating or locking connection. Also, a crank contact interlock must be provided to prevent the motor from operating while the crank is inserted, and until such contact has been reset.